The production of oil from underground reservoirs results in crude oil containing varying amounts of water generally in the form of a water-in-oil emulsion. It is general practice to dehydrate the crude oil by allowing it to stand but oftentimes the dehydration is enhanced by the addition of a demulsifier to break the emulsion facilitating physical separation of the crude oil from the water. Following this dehydration step, the crude oil is transported to the refinery where it may undergo an initial dewatering procedure and/or subjected to the process of desalting, i.e. the removal of salts from hydrocarbon crude oil, sometimes employing the action of an electrocoalescer.
Salts in hydrocarbon crude oil are generally dissolved in small droplets of water or brine dispersed throughout the crude. Sodium chloride is the primary salt followed by calcium chloride, magnesium chloride and the sulfates of these three metals. The total salt content ranges from substantially zero to several hundred pounds per thousand barrels of crude.
These brine droplets are generally prevented from coalescing and settling by a tough, elastic film at the surface of each droplet. This film is stabilized by natural emulsifiers found in the crude, solids, and solid hydrocarbons that concentrate at the droplet surface. A desalting chemical or demulsifier displaces these natural emulsifiers and solids and weakens the film so the droplets of brine can coalesce when they contact each other.
A new oil field will frequently produce crude with negligible water and salt. As production continues, the amount of water produced increases, raising the salt content of the crude. Additional salt contamination often occurs during tanker shipment. An empty tanker takes on sea water as ballast and often uses it to wash the tanks. To minimize pollution, the top, oily layer of ballast water and the washings are segregated in a slop compartment when the ballast water is discharged. Fresh crude is then loaded on top of this slop oil and water. The entire compartment is then offloaded at the refinery.
As earlier inferred, some brine can be removed by settling and water drawoff in the refinery's crude storage tanks. Some demulsifiers are very effective in increasing the rate and amount of settling as well as preventing sludge buildup and in cleaning tanks where sludge has already accumulated. Typically, the demulsifier formulation is injected into the turbulent crude flow as it fills the storage tank at a treat rate of from 10 to 500 ppm. The settled brine is drawn before the crude is charged to the pipestill.
The destructive effects of processing salt-contaminated hydrocarbon streams in refining operations have been well known for many years. These streams are heated for distillation or cracking effects and result in a decomposition of the salt into hydrochloric acid. Hydrochloric acid causes severe damage and lost onstream time in a refinery due to its very highly corrosive attack on metal processing equipment. Consequently, the removal of salt from crude oil (and its products) has been a major refining problem. A process was formed in the 1930's for the removal of the salt which contaminated hydrocarbon streams, such as crude oil. This process is described in U.S. Pat. No. 2,182,145. In this desalting process, the hydrocarbon stream is mixed with a small amount of fresh water (e.g. 10% by volume) forming a water-in-oil emulsion. The resulting emulsion is subjected to an electric field wherein the water is coalesced as an under flow from the upper flow of a relatively water-free, continuous hydrocarbon phase. The desalted hydrocarbon stream is produced at relatively low cost and has a very small residual salt content.
To enhance the effectiveness of electrostatic desalter, desalting chemicals are used in combination with an imposed electric field. Desalting chemicals are usually a blend of surface active materials in hydrocarbon solvents. These materials are preferentially absorbed at the brine droplet surface, displacing the solids and natural emulsifiers. This greatly weakens the film around the droplets. The brine droplets can then coalesce with the wash water (thus diluting the brine) and with other droplets so their size becomes large enough to settle by gravity. Depending on its composition and solvent, the desalting chemical may also dissolve the film.
To overcome solids stabilization of an emulsion, a good demulsifier formulation will cause the oil-wet solids to become water-wet and settle into the water phase where they are removed with the effluent water. A surfactant can also be used alone or in combination with the demulsifier for this purpose. These chemicals work by attaching an oil-loving or solids-loving section of the molecule to an oil-wetted solid. A water-loving section then physically drags the solid into the water phase. These molecules can also agglomerate solids to speed their settling. Without chemical treatment, most oil-wet solids will stay in the oil phase even though their density is higher.
A good demulsifier formulation will perform as follows. It will efficiently break the emulsion into oil and water phases. The rate will be fast enough in electrostatic desalting operations to prevent emulsion pad buildup which can short out the electrodes of the electrocoalescer and result in emulsified oil rather than an oil with reduced salt content going to the distillation tower and/or cause excessive oil carryunder. The water and salt will be removed from the oil within the residence time of the desalter. Minimal oil, i.e. known as oil carryunder, will be present in the effluent water which flows from the bottom of the coalescer. Solids will be water wet so they are similarly removed from the crude. Further the chemical must be able to treat many different crudes effectively. Finally the desalting system as formulated should not be a hazard to operations, e.g. it should have a flash point of at least 38.degree. C.
Both the dewatering and desalting demulsifier formulations must be sufficiently stable during storage and/or use that stratification of the formulation does not occur. Stratification is highly objectionable since it causes a drastic and unacceptible reduction of demulsification efficiency. Also highly objectionable for a demulsifier formulation is a tendency to foam since the presence of foam results in a decrease of effective operating capacity and/or increases the stability of the emulsion being treated. Further, the formulation must be cost effective.
It is, accordingly, the primary object of the present invention to obviate these and other prior art deficiencies, particularly by providing novel demulsifier formulations and processes for dewatering and/or desalting conventional whole heavy petroleum crudes, heavy petroleum crude fractions, residua, fuel oils and refinery hydrocarbon fractions (all of which are herein collectively called "hydrocarbon oil").